In the traditional system of cast-iron melting, the cast-iron is accumulated, in liquid form, in a blast-furnace hearth or crucible, and poured, at regular intervals, into ladles, casting machines, ingot forming machines, or sand molds. The quantity of cast-iron accumulated, being the maximum contained in the volume related to the diameter of the crucible and to the height of the tapping opening for the slag.
The traditional system, has the following major shortcomings:
(A) The crucible is too quickly emptied, causing a faster than the normal drop of the liquid column in the blast-furnace and consequently provoking variations in the blowing pressure, and the volume of injected air, as well as in the normal conditions of heat transfer and the chemical and physio-chemical balance between the solid, liquid and gaseous materials existing in the internal portions of the blast-furnace.
(B) Casting of the accumulated cast-iron is usually made six times in every twenty four hours. The opening of the metal pouring orifice (tapping hole) by means of a compressed air hammer, oxygen torch, chisel and/or hammer, constitues one of the more serious safety problems of the blast-furnace practice, especially in small size plants, where the cast-iron is poured directly into the sand molds, casting ladles or casting machine forms, without using intermediate pans. Furthermore, the regulation of the pouring of the liquid cast-iron, during the pouring operation, which regulation is an essential factor for obtaining a quiet and even filling of the sand molds or the forms, is practically unobtainable, and a violent flow of liquid cast-iron, when no pans are available for the storage of the liquid cast-iron, causes the irregular or improper filling of the molds with the consequent formation of large percent of scrap pig iron. It also causes bubbling and spraying of liquid metal over great distances, which is extremely dangerous and causes large losses. Added to this is the formation of large quantities of scrap iron during the pouring leakage, which constitutes a very serious economic factor, as this scrap when sold commands a much lower price than common pig iron.
(c) After the pouring of the pig iron from the blast-furnace, another serious problem is the need for adequately and promptly blocking the pouring holes of the furnace. In the large mills, such capping or blocking accomplished by expensive clay-ramming machines and associated equipment: in medium size and small mills, the work is done manually, exposing the workers to excessive heat and great risk of burns.
In any case, the opening, the closing and the maintenance of the pouring holes for the pig iron constitute major problems to the operators of blast-furnaces. Another major problem results from the damage caused by erosion of the pouring holes, and the adjacent refractory bricks. The furnace tapping beyond regular hours, also constitutes an equally serious problem in the operation of blast-furnaces.
(d) During the conventional, intermittent pouring of the melted pig iron, the level of the liquid pig iron in the blast-furnace crucible varies considerably during the time between two successive pourings; the same variation also occurring in the layer of slag which floats on the pig iron in the crucible. For that reason, the quantity of pig iron and slag accumulated in the crucible varies from minimum value (immediately after pouring) to a maximum value (immediately before the pouring of the pig iron or the slag). The increase in the level of these liquid layers accumulated in the crucible results, as a first consequence in a gradual increase in the blowing pressure of the blast-furnace. Such increase, if the mill does not possess constant pressure blowing equipment, diminishes the volume of air injected by the blowers, and consequently diminishes the rate of settling of the solid masses, and thereby causes a drop of the hourly production rate of the blast-furnace.
An excessive increase of the height of the level of slag in the crucible, may cause the slag to come in contact with the tuyeres, damaging or obstructing them. If the level of the smelted pig iron should reach the tuveres for the blowers, there is also the grave danger of explosion.
Since the refinement of the chemical composition of the pig iron takes place in the crucible, by the reaction of the pig-iron and slag and by the passing of the liquid pig iron through the layer of the slag, it is noted that the relative quantities (masses) of the pig iron and slag present have a great influence on the final chemical composition of the pig iron.
If, during the conventional process, the quantity of slag which floats on the pig iron varies from zero to the maximum value permitted, it is evident that the reaction of the pig iron with the slag, especially during the passing of the drops of pig iron through the slag, varies in efficiency during the interval between two successive pourings. This causes variations in the chemical qualities of pig iron accumulated in the crucible where it is separated by order of the density. The pig iron with lower silicon content tends to segregate at the bottom of the crucible.
In other words, during the same pouring of the smelted pig iron considerable variation in chemical analysis is obtained based on density segregation. This segregation of the pig iron in the crucible is compounded by the fact that there is practically no motion in the liquid mass, it being subjected to practically no agitation.